The Two Sister Study recruited women with young-onset breast cancer, an unaffected sister already being studied as part of the Sister Study cohort, and, when available, their parents. We have combined their data with the DNA and environmental data now being collected from their unaffected sisters (who previously joined the Sister Study) and saliva-based DNA collected from their parents. We are using a nuclear-family-based approach to study genetic and environmental factors involved in young-onset breast cancer. The study gained enormous operational efficiency advantages, by taking advantage of the infrastructure that was already in place and functioning smoothly for the Sister Study (Dale Sandler, PI). Follow-up of these cases (after merging with new cases in the Sister Study) will also allow us to identify environmental, clinical, and genetic factors that influence health after treatment. Case-parent analyses of gene variants are protected against bias due to confounding by genetic heritage, and also permit detection of both maternally-mediated genetic effects and parent-of-origin (imprinting) effects. In this study, the participating affected sisters each completed a computer-assisted telephone interview like the one their sister completed for the Sister Study, providing extensive information about personal exposures, reproductive history, and past occupational exposures. Nongenetic effects will be identifiable through a paired comparison of affected and unaffected sisters. Gene-by-exposure interactions will be assessed with novel statistical methods. In summary, the Two Sister Study leverages off the ongoing Sister Study to build a cost-effective, powerful, and statistically independent study of young-onset breast cancer. Findings related to combined effects of genetic variants and environmental factors can also be replicated in the Sister Study. With augmentation by including some newly diagnosed young-onset cases from the Sister Study we enrolled nearly 1500 cases providing both questionnaire data and DNA. We have also enrolled 1403 of their parents, who provided DNA. This work was accomplished with assistance from the EB support services contract. We secured permission from the funding agency (Susan G. Komen for the Cure) for a no-cost extension and for redirecting the money originally intended for a candidate gene approach to instead carry out a genome-wide association study (GWAS), using the Illumina OmniExpress plus Exome chip. Carried out through a contract with the Center for Inherited Disease Research, this genotyping project (augmented by imputations) generated more than 20 million SNPs on these families. We are also participating in the GAME-ON consortium, and this effort has provided additional genotype data based on the newly developed Onco-array chip, again through CIDR. We are using these data to find gene-by-environment causal factors for young onset breast cancer. By combining the Two Sister cases with those incident cases arising in the Sister Study we will also be able to study complexes of factors that are related to healthy recurrence-free survival following treatment. We have now published a paper reporting on the genome wide analyses, both for the maternal genotype and for the daughter-inherited genotype. There is interest in a possible relation between history of migraine headache and risk of breast cancer, and we carried out analyses related to that question using data from the Two Sister Study. We distinguished between migraines that tended to occur at a particular time of the menstrual cycle and those that did not follow a menstrual pattern. Overall there was no relationship between migraine and risk of young-onset breast cancer, but women with migraine who develop breast cancer and have a history of a menstrually-related migraine pattern are more likely to develop a hormone positive tumor. In work with a summer intern in 2015 we had evaluated the association between genital talc powder and use of douching in relation to incident ovarian cancer. Talc was not related, but the correlated behavior, vaginal douching, was associated with almost a doubled risk of invasive ovarian cancer. The paper was published and got the attention of the popular press. In work originated in the summer of 2014 with an intern, Denis Whelan, we considered self-reported history of a young-onset eating disorder in relation to demographic and other causes and also considered eating disorders at any stage of life as possible risk factors for breast cancer. We also described associations with later body habitus and reproductive outcomes. One paper is undergoing review and another is about to be submitted. An ongoing project collaborative with investigators at the Centers for Disease Control and Prevention is studying the participants in the Sister Study to ascertain their beliefs about risk and use of cancer screening and also studying the survivors of breast cancer to learn how their lives, their health and their outlook have been changed by the diagnosis of breast cancer.